This application claims the priority of German patent 197 33 474.1-14, filed Aug. 2, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for manufacturing secondary mold elements such as necks, protrusions, raised elongate flat areas, and the like on elongate, circumferentially closed hollow shapes by means of internal high-pressure forming, and an apparatus for performing such a method.
A method and an apparatus of the type generally described above are known from German patent document DE 94 07 812.2 U1. In this document, a secondary mold element in the form of a hollow cylindrical neck is produced on a tubular hollow shape, with the hollow shape being placed in a bipartite internal-high-pressure mold and, after the tool is closed, being subjected internally to a high-pressure fluid generated by an externally located high-pressure fluid generating system. The cavity in the mold that matches the contour of the hollow shape blank shapewise has a branch into which the hollow shape material is forced under the influence of the internal high pressure together with an axial upsetting movement (i.e., a pressing-in movement) that follows the hollow shape material, by endwise impact on the hollow shape by two pressure pistons displaced axially opposite one another back and forth in the cavity, forming a neck. In order to prevent premature bursting of the hollow shape material in the shaping area of the neck, a counter plunger is displaceably guided in the branch, said plunger abutting the hollow shape during the entire shaping process and impacting the hollow shape with a force that is directed opposite the force of the expansively acting internal high pressure, with the counter plunger deflecting outward under control so that the neck is not formed suddenly but under control. At the same time, the counter plunger ensures that the hollow shape material does not thin out excessively in the cap area of the neck so that shaping can proceed safely up to a certain neck height. This neck height depends on the thinning of the material of the hollow shape in the mouth area of the branch to the cavity, since stretching takes place at this point, even with a large radius for the bending edge at this location, and also depends upon the accumulation of material in the abovementioned cap area. The method described, for a safe manufacturing procedure, applies exclusively to short hollow shapes since in the case of long hollow shapes, in the range from approximately one meter and more, the frictional forces between the wall of the cavity and the hollow shape during the axial pushing indicated become so great that this is no longer possible without undesired folding of the shape and/or shearing cracks in the shape. In long hollow shapes, therefore, necks can only be produced by a simple expansion process, but with only the material present in advance in the neck area being available and this therefore can only be obtained from the wall thickness of the hollow shape. A clearly visible elongate neck is therefore not possible.
A goal of the invention is to improve on a method and a device for forming secondary mold elements such that expansion of the method limits during the manufacture of a secondary mold element with respect to an increase in its height can be achieved in a safe and simple manner.
This and other goals have been achieved according to the invention by providing a method for producing secondary mold elements as necks, protrusions, raised elongate flat locations, and the like on elongate, circumferentially closed hollow sections by means of internal high-pressure shaping, with the secondary mold element being shaped locally by the expansion of the hollow section using a high-pressure fluid therein, and being supported under control during the shaping process by a counter plunger that yields outward with increasing shaping, characterized in that hollow section is upset (i.e., pressed in) at the location of the secondary mold element to be produced by counter plunger, whereupon the hollow shaped material in area of inward upsetting is pressed circumferentially and endwise by the high-pressure fluid at plunger and that after the upsetting process, by retraction of plunger which constantly abuts hollow section during the entire shaping process, in conjunction with the internal high pressure, hollow section is externally upset and secondary mold element is formed.
This and other goals have been achieved according to the invention by providing an apparatus for producing secondary mold elements such as necks, protrusions, raised elongate flat locations and the like on elongate, circumferentially closed hollow sections, with an internal high pressure mold divided into an upper tool and a lower tool, whose cavity that receives a hollow section has at least one branch in which a counter plunger provided with a controlled drive is displaceably guided, and with a high-pressure fluid system that is connectable fluidically with internal high pressure mold, characterized in that counter plunger is guided in a passageway of a stop body that is rigidly located in the branch and abuts branch wall closely in such fashion that counter plunger can be displaced into cavity for inward upsetting of hollow profile located with limited play in cavity of mold, and can be sunk under the influence of internal high pressure into stop body for shaping secondary mold element.
According to the invention, as a result of the internal high-pressure-produced contact between the hollow shape and the counter plunger in the upsetting (i.e., pressing-in) position of the counter plunger, hollow shape material is displaced into the branch area and the maximum material stress takes place in the fibers of the material that will subsequently be subjected to less stress. As a result, firstly, in the shaping of the secondary mold element, the hollow shape is relieved of a load in the mouth area of the branch, since at that point, as a result of the material accumulated in the branch area in advance, following the initial projection of the hollow shape by pulling back the counter plunger, it is only considerably later that the conventional widening, and hence the material thinning, occurs in the wall of the hollow section. At the same time, because of the increased accumulation of material in the branch area, the cap area of the secondary mold element also grows much thinner later, so that with the same state of dilution of the hollow section material at the critical points in the secondary mold element, the mouth area of the branch and the cap area, by comparison with conventional methods for producing secondary mold elements, greater heights for the secondary mold element during its shaping can be achieved without cracks occurring at these points and hence without the hollow shape bursting in the branch. Consequently, the method limits in the manufacture of a secondary mold element regarding an increase in its height are increased and the safety of the shaping process is ensured. In this way, the reject rate in manufacturing hollow shapes with secondary mold elements can also be significantly reduced.
The formation of the increased height of the secondary mold element is made possible in simple fashion since the counter plunger, as a result of constant supporting contact with the hollow shape for controlled shaping of the secondary mold element, is present in any event, it is used for internal upsetting and hence for accumulating material. The secondary mold element is shaped by expanding the hollow shape using a high-pressure fluid generated therein, whereby the counter plunger, yielding outward with increasing shaping, supports the hollow shape in the shaping area during the shaping process under control.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.